1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to liquid crystal display devices applied with specific orienting films.
2. Description of the Prior Art
Liquid crystal display devices are produced usually by preparing a pair of glass plates bearing transparent electrodes arranged in the pattern of picture elements, spacing the glass plates face-to-face at a distance of 5 .mu.m to 100 .mu.m to obtain a sandwich cell, placing a liquid crystal composition into the cell and sealing the cell. The optical characteristics of the liquid crystal cell are dependent largely on the orientation of the liquid crystal within the cell.
Especially with liquid crystal display devices which operate under the action of an electric field (FEM liquid crystal display devices), it is required to show a uniform liquid crystal orientation. The orientation of the liquid crystal is usually controlled by a surface treatment of the cell, or by adding a very small amount of a chemical substance to the liquid crystal.
As a mode of orientation, the liquid crystal molecules are aligned with their axes parallel to the substrate plane (homogeneous alignment) by rubbing the surface of the substrate with paper or fibers, or by forming with a slant evaporation an orienting film of vacuum-evaporated SiO.sub.2 on the substrate surface, while research has been conducted in recent years on the method of obtaining the desired orientation by preparing an orienting film of an organic high polymer and rubbing the film. This method appears very useful in view of the uniformity of the orientation available and the simplicity of the procedure.
It has been known that, inorganic films include SiO.sub.2, TiO.sub.2 and Y.sub.2 O.sub.5, and organic high polymers include polyimide, polyimide-isoindoloquinazolinedione, polyimide-benzoimidazopyrrolone, polyvinyl alcohol (P V A) and polyamide to prepare orienting films for FEM liquid crystal display devices.
The orienting films prepared from such polymers followed by rubbing treatment induce a uniform orientation on liquid crystals, have high heat resistance and good insulating properties and will not readily permit erosion of the orientation even when heated to 100.degree. to 150.degree. C. for sealing.
However, dielectric constants of such conventional films of organic high polymers are varied dependent on a frequency of applied signals thereto. The dielectric constants become larger as the frequency becomes lower. This is because the polarization of a dielectric film made of an organic high polymer appears as electronic polarization in a high frequency range, but in a low frequency range the polarization appears as molecular polarization, orientation polarization and interfacial polarization so that the dielectric constant becomes larger.
When a liquid crystal display having an orienting film made of an organic high polymer with a large dielectric constant is driven with an AC voltage, the impedance of the orienting film is small so that a negligible voltage drop may be provided with the orienting film. However, since the liquid crystal contains inevitably ionized impurities, they are attracted toward the surface of the orienting film to form an electric double layer. The orienting film is prevented from polarizing, the voltage drop due to the orienting film can not be neglected. In particular, the conventional liquid crystal display is driven with low frequency signals of about 100 Hz so that the voltage drop becomes serious.
The ionized impurities for causing the electric double layer are ions (Na.sup.+ and K.sup.+) separated from the surface of a glass flask used to manufacture and compose the liquid crystal. Other ions such as Fe.sup.++, Na.sup.+, Cu.sup.++, H.sup.+, OH.sup.-, Cl etc. may be contained as the inonized impurities.
When an electric field is applied across the orienting films and the liquid crystal containing the ionized impurities, the ions in the liquid crystal tend to gather round the surfaces of the orienting films to thereby reduce the dielectric constants of the orienting films and form the electric double layer. Hence, the voltage drop appears outside the liquid crystal layer and an actual voltage to be applied across the liquid crystal layer is lowered. This increases the threshold voltage of the liquid crystal and decreases the display contrast.